This invention relates to a metal-based cubane structure contained in an octanuclear complex which can be used as an paramagnetic agent and/or an electron-transfer agent. Paramagnetic compounds are useful for a wide variety of applications. One particular use is as a contrast agent for magnetic resonance imaging (MRI). Generally the contrast agent is injected or otherwise diffused within the subject of observation. When the target is then scanned by an MRI machine, the areas containing the contrast agent appear brighter (or darker) because of its paramagnetic properties. Electron-transfer agents also are useful for a wide variety of applications. Some particular uses are: as molecular junctions, oxidation catalysts, electron acceptors in chemosensors and photovoltaic systems.
More specifically, this invention is directed to a redox-active metal-based structure, protected inside an inert coating. This complex, which can be defined as Fe8(μ4-O)4(μ-4-R-pz)12L4, is stable over several oxidation states. The present invention is also directed to a method of making this product from simple starting materials.
Thermodynamic stability is a desired property of materials to be used in most commercial applications. At the same time, however, chemical versatility is typically required for the manifestation of interesting properties or catalytic activity. A combination of these contrasting characteristics is typically achieved by the coating of large surfaces or particles with inert substances, at the macroscopic and microscopic scale, respectively. Such structures are known to naturally occur at the macromolecular level when, for example, metal active centers are protected inside the organic part of metalloproteins.
The complex encompassed by the present invention has a core, which is the source of redox properties, encapsulated in a protective shell. Such a complex provides an ideal building block for the construction of one-, two- and three-dimensional materials. These materials can be constructed by connecting or bonding units of the complex made in accordance with the present invention by bridging atoms or groups. The bridges can be either bidentate ligands, which replace terminal atoms, or bifunctional substituents, which connect the units through substitution at the 3-, 4- or 5-position of their respective pyrazoles. The advantage of using the inventive complex for the construction of these materials, instead of a single metal atom, or other mono- or polynuclear products, is that the complex can withstand redox manipulation without significant geometrical changes, which would cause the structure of the material to collapse. The invention discovers that the structural integrity of the present complex is a function of the way it is composed. Specifically, the desired redox properties are a function of the metal core, while the connections required for the construction of the above proposed materials take place at the outer inert shell. Accordingly, construction of 1-, 2- and 3-dimensional materials leaves the core unaffected. Similarly, redox changes in the core leave the outer structure unaffected. In addition, when the metal atoms employed are paramagnetic, the complexes encompassed in the invention are also paramagnetic, or can become paramagnetic in one of their oxidation states. The materials which will be prepared from the inventive complex will have all the magnetic and redox properties of this building unit, possibly even amplified.
The invention surprisingly and unexpectedly demonstrates that by encapsulating a redox active core inside an inert protective coating, the resulting material retains structural stability over several oxidation states. This result is achieved by separating the center of redox activity, the core, from the outer surface of the molecule, the coating. In some cases, the molecular symmetry of the complexes Fe8(μ4-O)4(μ-4-R-pz)12L4 allows the existence of optically active forms, which can be prepared as racemic mixtures or enantiomerically enriched or enantiomerically pure forms.